1. Field of the Invention
The invention relates generally to threaded tubular joints used in oil and gas well drilling and production, such as tubing, casing, line pipe, and drill pipe, commonly known collectively as oilfield tubular goods. Particularly, the invention relates to a tubular joint for connecting male (pin) and female (box) members by relative rotation and the application of torque for makeup. More particularly, the invention relates to a two-step wedge thread having load flanks, stab flanks, and a positive-stop torque shoulder that will provide a secure and pressure sealed connection between male (pin) and female (box) members of tubular joints without applying excessive torque and work energy for makeup.
2. Background Art
The use of threaded tubular connections for joining flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid under pressure is well known. One particular use of oilfield tubular members is for drilling a borehole to a desired depth by joining together sections of the tubular members. The joints are intended to support both compression and tension loads, to transmit rotation forces, or torque, from one member to the next, and to seal a passage for pressurized fluid to be transmitted through the interior of the tubular members. Oilfield tubular goods typically use threaded end connections or joints for connecting adjacent sections of conduit, pipe or tubular members. Examples of such threaded end connections designed for use on oilfield tubular goods are disclosed in U.S. Pat. Nos. 2,239,942; 2,992,019; 3,359,013; RE 30,647; and RE 34,467, all of which are assigned to the assignee of the present invention.
In U.S. Pat. No. RE 30,647 by Blose, a particular thread form or structure is disclosed for a tubular connection that provides an unusually strong joint while controlling the stress and strain in connected pin and box members within acceptable levels. The pin member is equipped with at least one generally dovetail-shaped external thread whose width increases in one direction along the pin, while the box member is equipped with at least one matching generally dovetail-shaped internal thread whose width increases in the other direction. In this manner, the mating set of helical threads provide a wedge-like engagement of opposing pin and box flanks that limit the extent of relative rotation between the pin and box members, and define a forcible makeup condition that completes the connection. This is called a “wedge thread.” In this wedge thread structure, the flank angles as well as the thread width can be used to control the stress and strain preload conditions induced in the pin and box members for a given makeup torque. Thus, by tailoring the wedge thread structure to a particular application or use, the tubular connection or joint is limited only by the properties of the materials selected.
It will be understood that certain terms are used herein as they would be conventionally understood where tubular joints are being connected in a vertical position along a central axis of the tubular members such as when making up a pipe string for lowering into a well bore. Thus, the term “load flank” designates the side wall surface of a thread that faces away from the outer end of the respective pin or box member on which the thread is formed and supports the weight (i.e., tensile load) of the lower tubular member hanging in the well bore. The term “stab flank” designates the side wall surface of the thread that faces toward the outer end of the respective pin or box member and supports forces compressing the joints toward each other such as the weight of the upper tubular member during the initial makeup of the joint or such as a force applied to push a lower tubular member against the bottom of a bore hole (i.e., compressive force). The term “face” of the box is the end of the box member facing outward from the box threads and the term “nose” of the pin is the end of the pin member facing outward from the threads of the connection. Upon makeup of a connection the nose of the pin is stabbed into and past the face of the box.
As shown in FIG. 1, a single-step prior art connection 10 includes a pin member 11 and a box member 12. Box member 12 has a tapered, internal thread structure 14 formed thereon and adapted for engaging complementary tapered, external thread structure 15 formed on pin member 11 to mechanically secure the box and pin members in a releasable manner.
Internal thread 14 of box member 12 has stab flanks 18, load flanks 16, roots 20, and crests 24. The box thread 14 is a wedge thread that increases in width progressively at a uniform rate in one direction substantially the entire helical length of thread 14. External thread 15 of pin member 11 has stab flanks 19, load flanks 17, roots 21, and crests 25. The pin thread 15 is a wedge thread that increases in width progressively at a uniform rate in the other direction substantially the entire helical length of thread 15. The oppositely increasing thread widths and the taper of the box and pin threads 14 and 15, respectively, cause the complementary flanks, roots, and crests of the respective threads to move simultaneously into forcible engagement during rotational makeup of the connection. The wedge threads may have a rectangular shape cross-section, a dovetail shape cross-section, or another shape continuously along the helical length of the progressively tapered wedge thread. Upon rotational makeup of the connection, surface-to-surface engagement of the threads can provide sealing surfaces that resist the flow of fluids between the threads. In a well formed wedge thread connection a thread seal is provided. An additional seal could be provided at makeup, as for example with a metal-to-metal seal formed by radial interference between overlapping portions of the ends of the connection, as at a tapered portion 26 of the pin member 11 and a tapered internal portion 27 of the box member 12.
The pin member 11 or the box member 12 defines a longitudinal axis 13 of the made up connection 10. The roots and crests of the box and pin members are generally flat and parallel to the longitudinal axis 13 of the connection and have sufficient width to prevent any permanent deformation of the threads when the connection is made up. For example, having a minimum thread width at the root of the thread that is greater than the height of the thread can generally provide adequate shear area to support the flanks under load. A taper is created by the diameter of the roots and crests of the external thread 15 progressively increasing from the nose of the pin member 11 into the connection, and by the diameter of the crest and roots of the internal thread 16 progressively decreasing from the face of box member 12 into the connection.
FIG. 2 shows a prior art two-step wedge thread connection 28. A pin member 29 is in threaded engagement with a box member 30 to form the connection 28 co-axially along a central axis 40. The threads that comprise the connection are separated on two different “steps,” a large step indicated by the bracket 31 and a small step indicated by the bracket 32. The crests 33 of the threads in small step 32 of the pin member 29, at their full design height, do not interfere with the crests 32 of the threads in the large step 31 of the box member 30 when the pin member 29 is “stabbed” into the box member 30. The small step 32 of the pin member 29 is smaller than the smallest crest-to-crest thread diameter in the large step 31 of the box member 30. The small pin external threads can be “stabbed” past the large box external threads and the number of engaged threads. Both the small and large threads engage with each revolution to makeup the connection. Thus, the number of revolutions during which the threads slide or rub against each other is reduced for the same number of engaged threads.
Typically, connections will be designed to include metal-to-metal seals for keeping the conduit fluid pressure tight at the connections. Generally speaking, metal-to-metal seals are created when contact pressure between two metal surfaces exceeds the fluid pressure to be sealed along a continuous contact area circumferentially around a connection. The contact pressure is generally created during makeup of the connection, although some types of metal-to-metal seals are additionally energized by internally pressurizing the conduit. Metal-to-metal seals have the advantage of not suffer degradation from high temperatures or chemicals often found in a well. Smooth, uniform surfaces facilitate making a seal. Alternatively, making a seal may be facilitated by providing sufficient force to deform and compress together discontinuities to form a continuous circumferential contact pressure area between opposed pin and box surfaces. Positive-stop torque shoulders are sometimes used for this purpose.
FIG. 2, which was discussed above, shows an embodiment of a tubular joint with metal-to-metal seal 34 is located between the large step 31 and the small step 30. The seal 34 is created by interference between metal surfaces of annular shoulders at 39 on the pin member 29 and the box member 30 and the resulting metal-to-metal contact pressure. The surfaces of the annular shoulders elastically deform a slight amount in compression providing surface-to-surface sealing engagement while the interengaged threads at the two steps effectively provide a positive stop against the applied torque. A metal-to-metal seal is also shown at the nose 35 of the pin member 29. The complimentary surfaces are pushed together by the axial force generated by engagement and rotation of the load flanks when the connection 28 is made-up. It will also be understood that a metal-to-metal seal could also be formed at the base end 36 of the box member 30 of the connection. Thus, metal-to-metal seals may be formed at either end 35 or 36, or in the middle 34 of a two-step connection or at any combination of more than one of the locations 34, 35, and/or 36.
A threaded connection having one or more negative angle load flank can facilitate sealing at the connection. A negative angle flank is a flank that is not perpendicular to the centerline of the connection, but instead has an angle that creates a trapping mechanism with a complimentary flank. For example, in a thread with a dovetail cross-sectional shape, oppositely directed radial forces are generated by negatively angled flanks between the internal and external dovetail threads. The oppositely directed radial forces pull the internal and external threads together. If the internal and external threads have corresponding sizes and shapes, the trapping action can create sufficient surface-to-surface contact pressure between adjacent surfaces, between the roots and crests, and/or between load flanks and stab flanks, to effect a pressure seal at the connection.
While negative flank angles can be beneficial for certain sealing purposes, any frictional drag caused by the sliding or rubbing of one surface against another under a contact force or pressure must be overcome by the rotational torque when rotating the tubular members to makeup the connection. Completing the makeup of a connection with a large surface-to-surface contact pressure, or with a large makeup contact area, requires a large amount of torque. A large contact pressure for a given area or a larger the contact area for a given contact pressure requires a correspondingly large torque for makeup. As connection diameters increase, the thread rubbing area increases such that the same amount of surface-to-surface contact pressure will require a greater amount of torque. Also, the greater the number of revolutions required to complete makeup, under the same frictional contact conditions between threads, the greater the input work and energy required.
In U.S. Pat. Nos. 6,174,001 and 6,270,127 by Enderle, two-step, low torque wedge threads for tubular connectors are disclosed. One of the steps is provided so that there is interference contact at makeup along at least one of the complementary stab flanks, load flanks, roots and crests and so that clearance is provided along another step at least along one of the complementary stab flanks, load flanks, roots and crests for reducing the amount of torque for makeup of the connection while retaining torque sensitivity, sealing capability, and threads necessary for structural purposes. In the wedge thread arts there continues to be a need for development of desirable relationships between the contacting portions and the clearance portions and for additionally providing specific bases and criterion for determining useful relationships between surfaces of the wedge threads that make contact at makeup and the surfaces that are provided with clearance.